nouveau: misc fixes from upstream + NVAF support
    
    This looks far far worse than it is... The reason:
     - NVAF support came after nv50_grctx.c was de-magiced (ie. lots of
       comments explaining what all the hexified numbers are for)
     - nv50_grctx.c de-magic depended on an earlier commit which changed
       all gpu object accessors from 32-bit indices to byte offsets
    
    All in all, it was easier to pull in those big scary-looking (but
    harmless) commits than to risk messing up a backport.